Method and structure for encapsulating electric apparatus



Aug. 23, 1960 Filed May 14, 1956 C E. MERCIER METHOD AND STRUCTURE FOR EINCAPSULATING ELECTRIC APPARATUS 25 all Z6 2 Sheets-Sheet l Aug. 23, 1960 c. E. MERCIER 2,950,339

METHOD AND STRUCTURE FOR ENCAPSULATING A ELECTRIC APPARATUS Filed May 14, 1956 2 Sheets-Sheet 2 Elma Mort g- Bot AX @IZvuoic/L United States Patent METHGE) AND STRUCTURE FOR EN CAPSULAT- ING ELECTRIC APPARATUS Carl E. Mereier, West llis, Wis., assignor to Allis- Chalmers Manufacturing Company, Milwaukee, Wis.

Filed May 1'4, 1956, Ser. No. 584,695

2 Claims. (Cl. 174102) This invention relates to electric structures and more particularly to electric induction apparatus and the process of encapsulating this apparatus with a given material.

Insulation is a necessary component of electrical equipment and the application and quality of such insulation affects its operational characteristics by rendering it either useful or useless in many situations. Heretofore, transformers were insulated by hand wrapping layers of insulation and insulation tape around them, or by encapsulating with resins molded under pressure. The method of insulating these devices and the combination of component parts used in them often resulted in the resin shrinking unevenly thereby causing cracks, air voids, stresses and strains which either lowered or destroyed the insulating effectiveness of the resin. These weaknesses in the encapsulating resin occurred during curing of the resin or use of the structure.

Tests have shown that core and coil assemblies with cast insulation may be manufactured without cracks or voids by placing adjacent the encapsulated parts of the device a layer or more of resilient material such as tape which is yieldable and expandable during both the casting and the curing operations. This resilient material may be semiconductive or conductive thereby lowering the electric potential difference existing across any air gaps or voids occurring between the resilient material and the wrapped member. If tape is semiconducting or conducting it is placed on the conductor so as to avoid a short circuit turn.

Therefore, in accordance with the present invention a new and improved method and structure is provided. The method comprises wrapping an electrical conductor with a first layer of resilient material, forming when in the pliable state a second layer of thermosetting insulating material substantially around the electrical conductor and curing the insulating material to form a shield of uniform density substantially around the conductor.

The structure comprises a current conducting member, a first layer of resilient material formed substantially around the conducting member, and a second layer of thermosetting insulating material formed when in the pliable state substantially around the resilient material. The resilient material yields during curing of the insulat ing material to retain the physical uniformity of the insulating material.

It is, therefore, one object of the present invention to provide a new and improved method of insulating an electric device.

Another object of this invention is to provide a new and improved method of encapsulating components of electromagnetic inductive apparatus.

A further object of this invention is to provide a new and improved method of eliminating radial cracks, voids, uneven shrinkage, and other conditions which reduce the dielectric integrity of the encapsulating material.

A still further object of this invention is to provide a new and improved substantially encapsulated electric device in which the dielectric integrity of the encapsulating material is improved.

ice

A still further object of this invention is to provide a new and improved substantially encapsulated electric structure in which radial cracks, air spaces, and uneven shrinkage in the cured encapsulating material are eliminated.

A still further object of this invention is to provide an encapsulated electric structure in which certain components of the structure are wrapped with resilient material which compresses and expands during the casting and curing processes of the encapsulating material, there by eliminating radial cracks, uneven shrinkage and voids.

A still further object of this invention is to provide an insulated electrical apparatus in which the insulation has physical toughness, high moisture resistance, and high insulation properties.

Other objects and advantages of the invention will become apparent from the following description when read in connection with the accompanying drawings, in which:

Fig. l is a view in cross section of an encapsulated current transformer embodying one version of the invention;

Fig. 2 is a sectional view of Fig. 1 taken along the line lL-H;

Fig. 3 is a partial view in cross section of a modification of the structure shown in Fig. 1;

Fig. 4 is a view in cross section of a further modification of the structure shown in Fig. l; and

Fig. 5 is an end view of the structure shown in Fig. 4.

Referring more particularly to the drawings by characters of reference, Fig. 1 illustrates a wound type current transformer 9 embodying one version of the invention. Current transformer 9 comprises a core 10 mounted on a pair of brackets 11, a secondary winding 12 mounted on one leg of core 10 and magnetically coupled therewith, and a primary conductor 13 disposed about secondary winding 12 and spaced apart therefrom by resinous, encapsulating material 16. A continuous layer of insulation 18 separates the layers of primary conductor 13.

Primary conductor 13 comprises a spirally wound bar stock coil 20 of one or more turns having a beginning 22, an end 23, a first terminal 24, a second terminal 25 and a juncture member 26. Iuncture member 26 is mechanically and electrically connected to lead-in terminal 24. The electrical and mechanical connections between terminals 24 and 25, juncture member 26 and primary coil "20 may be obtained by resistance brazing or any other suitable means. A strip of insulation 28 isolates lead-in terminal 24 and juncture member 26 from end 23 of conductor 13 and from terminal 25.

The current flow, assuming it to be from lead-in terminal 24 to terminal 25 of conductor 13, will flow from lead-in terminal "24 through juncture member 26, from beginning 22 to end 23 to lead-out terminal 25. The current flow in primary conductor 13 induces a voltage in secondary winding 12 which will produce a comparatively low current measurable by a suitable indicator such as an ammeter (not shown) serially connected therewith in a manner well known in the art.

in accordance with the method and structure claimed, a thin layer of resilient, yieldable material 36 is wrapped substantially around the primary conductor 3.3 shown in Fig. 1. Resilient material 3t) may comprise, for example, machine or hand wrapped silicone rubber formed in one or more layers around primary conductor 13. Silicone rubber can be applied in the form of tape but it also can be applied by brushing or spraying with a liquid form of this material. Conductor 13 can be clipped in a silicone rubber solution if so desired.

A thicker layer of thermosetting insulating material 16 is formed around primary conductor 13 outside of resil- Patentecl Aug. 23, i960 ient material 30, substantially enclosing primary conductor 13 in the manner illustrated in Fig. 1. The layer of encapsulating material 16 is formed when in a pliable state substantially around resilient layer 30 outside of primary conductor 13 and within all spaces between core and secondary windinglz, and between the outside circumference of secondary winding 12 and the insidecircumference of primary conductor 13. One method of so forming this layer of thermosetting insulating material around primary conductor 13 and the other parts of transformer 9 comprises the steps of placing primary conductor 13 (with the layer of resilient material 30 surrounding it) and the other component parts of assembled current'transformer 9 Within a mold (not shown), filling the mold with encapsulating material such as a thermosetting resin under vacuum conditions and curing the encapsulating material to bind the component parts of the transformer together and to form a hard insulating encapsulating shell substantially around the device.

During the process of curing the encapsulating resin, stresses and strains are created which heretofore cracked and deformed the resinous material and destroyed its dielectric integrity. It is to eliminate these conditions that a layer of resilient material 30 is wrapped around the encapsulated parts. Material 30 which may be both porous and resilient,responds during curing to the expansion or contraction of encapsulating material 16. Resilient material 30 by expanding and contracting during hardening of thermosetting insulating material 16 provides relief means for the stresses and strains to be transmitted from one portion of the resin to another without cracking or creating voids. Encapsulating layer 16 thus has a virtually uniform densit substantially free from such faults as radial cracks, uneven shrinkage and voids. a

Particular processes of curing the layer of encapsulating material may be utilized in connection with particular insulating materials, but the present invention is'not dependent on or limited to any given curing process.

Encapsulating material 16 may be of any suitable insulating material, one embodiment of the present invention utilizing an epoxy type resin. Minute openings 32 at the ends of encapsulating material 15 may serve to make the layer of material 30 more resilient by providing a passage means for the resilient material 30 to breathe,

or in other Words to receive or expel air, but are not necessary to the proper functioning of the invention.

Fig. 3 shows the primary winding and terminals of a wound type current transformer embodying another version of the present invention with underlying structure similar to the structure illustrated in Fig. 1. The core and secondary winding have been omitted for the sake of clearness from the structure shown in Fig. 3 but it is assumed that the encapsulating material covers the structure in the same manner shown in Fig. 1. Primary conductor 13 is wound with a yieldable, resilient layer of tape 35 which can be insulating or semi-conducting, a

layer of copper mesh 36 loosely wound around tape 35,

and another layer of tape 35 wrapped around mesh 36.

The mesh 36 is so placed on conductor 13 as not to form a shorted turn. The copper mesh 36 individually or in combination with the tape 35 gives the necessary resiliency for absorbing the stresses and strains of curing resin 16 in the manner described above, under the description of Figs. 1 and 2. The application by hand or machine wrappings of the tape 35, copper mesh 36 and tape 35 precedes the formation of a thicker layer of encapsulating material 37 substantially around the resilient layers of tape and wire mesh. The encapsulating material 37, which is added over the tape and wire mesh, may be of any suitable insulating thermosetting material, such as an epoxy type resin which polymerizes or cures with or without further treatment after casting or molding. The resilient layers of material composed of layers 35 and 36 yield under pressure during the curing process and thus allow encapsulating material 37 to expand in both directions from its radial center. Again, minute openings 38 may be utilized at the ends of encapsulating material 37 to allow air to enter or escape, thereby permitting breathing during and after the curing process, but are not necessary to the proper functioning of the invention since the compression and expansion of the air or gas captured between the layers of resilient material 30 may be satisfactory for providing the medium for expanding and contracting during the curing of resinous material 37.

Although copper mesh 36 has been illustrated in the structure shown in Fig. 3, it is nevertheless intended to be within the scope of this invention to utilize any form of wire or mesh either conductive or nonconductive which has the desired resiliency or yieldability.

Fig. 4 is a view, in cross section, of a modification of the encapsulated current transformer illustrated in Figs. 1 and 2 wherein like parts are provided with corresponding reference characters.

A layer of resilient material such as silicone rubber tape 39 is wrapped substantially around primary con- 'ductor 13, secondary winding 12 and core 10. These layers of resilient material 39 are ordinarily applied by hand or machine wrapping, but other suitable methods such as dipping, painting or spraying may be used.

A layer of thermosetting material 40 substantially encapsulates the entire transformer with the exception of terminals 24 and 25 and the ends of brackets 11. Encapsulating material 40 fills substantially all of the re cesses to form a solid mass.

. Encapsulating layer 40 of thermosetting insulating material is formed when in the pliable state by a process of vacuum casting or any other suitable casting or molding method. During subsequent curing or polymerization of the resinous material the resilient layers of tape or mesh respond to the expansion or contraction of the encapsulating material to provide a means for the encapsulating layer of resin 40 to form a shield of uniform density substantially around the transformer.

Fig. 5 is a side view of the encapsulated current transformer illustrated in Fig. 4.

Although but a few embodiments of the present invention have been illustrated and described, it will be apparent to those skilled in the art-that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. The combination comprising a current conducting member, a first layer of resilient tape formed substantially around said member, a second layer of resilient mesh formed around said first layer of resilient tape, a third layer of resilient tape formed around said second layer of resilient mesh, and a fourth layer of theimosetting insulating material arranged substantially around said third layer of resilient tape.

2. The combination comprising a current conducting member, a first layer of resilient tape formed substantially around said member, a second layer of copper mesh formed around said first layer of resilient tape, a third layer of resilient tape formed around said'second layer of copper mesh, and a fourth layer of thermosetting insulating material arranged'substantially around said third layer of resilient tape.

References Cited in the file of this patent UNITED STATES PATENTS 

